DEGRADATION OF PROTEASOMES BY LYSOSOMES IN RAT-LIVER

Proteasomes are high-molecular-mass multisubunit complexes which are b elieved, either by themselves or as a part of the 26S proteinase compl ex, to play a central role in extralysosomal pathways of intracellular protein breakdown. We have addressed the degradation of proteasomes i n rat liver, investigating the possible role of lysosomes. Affinity-pu rified antibodies against rat liver proteasomes were used for immunobl ot analysis of isolated lysosomes. Although proteasomes are not found in lysosomes from normally fed rats, they were found to accumulate in lysosomes of rats treated with leupeptin (an inhibitor of lysosomal pr oteases) and could also be detected in lysosomes isolated from livers of starved (24 h) rats. Proteinase-K treatment of these fractions, as well as immunogold procedures, show that a proportion of the proteasom es are inside lysosomes. Comparison of the amount of proteasomes found in lysosomes by immunoblotting with their experimentally determined h alf life (8.3 days) is consistent with an important role of these orga nelles in the degradation of rat liver proteasomes. Nevertheless, thes e data do not exclude the possibility that some nonlysosomal degradati on of proteasome components also occurs. Since proteasomes were locali zed in autophagic vacuoles, it is likely that they are taken up mainly by nonselective autophagy. However, using an in vitro system, it was found that, under conditions of starvation, proteasomes may also be ta ken up into lysosomes and degraded via the heat-shock cognate protein of 73 kDa (hsc73)-mediated transport.